The present invention relates to a driveline system for a four wheel drive vehicle, and more particularly, to a differential device for use in such a driveline system. The use of the term "differential" does not imply the presence of conventional differential gearing, but instead, the term is used primarily because the device of the present invention replaces the typical prior art center differential. However, it should be understood that the device illustrated, described and claimed herein has uses other than as a center differential.
In many of the vehicles being produced today, the basic vehicle platform is a front wheel drive. However, in many such vehicles, especially in vans and sport utility vehicles, it is considered desirable to be able to provide four wheel drive, at least under certain operating conditions.
Typically, the various arrangements for achieving part-time four wheel drive have been complex and expensive, and in some cases, have required some sort of control scheme to achieve the four wheel drive (or rear wheel drive) in response to certain predetermined operating conditions.
It is known from U.S. Pat. No. 5,070,975 and from European Application EP 0 314 420 to use, as a center differential in a four wheel drive type of driveline, a viscous actuated ball ramp type friction clutch to transmit torque to the rear axles when there is a speed difference between the front and rear wheels. It is believed that the torque transmitting capability of the device of the cited patent and application would be somewhat limited by the fact that one of the ball ramp plates comprises the input to the viscous coupling. Despite such shortcomings, the device of the cited patents and application would be at least generally functional when transmitting a "positive" torque, i.e., whenever the vehicle is moving in a forward direction, and torque is being transmitted from the input, through the center differential into the output.
However, in most vehicle applications, it is necessary for the center differential device to be able to deal with "negative" torques, i.e., situations in which, with the vehicle still moving forward, torque is transmitted from the rear axles to the front axles. For example, because such vehicles typically have more weight on the front wheels than on the rear wheels, more braking effort is expended at the front wheels than at the rear wheels. Therefore, during a braking operation, using merely a standard center differential, negative torque would be transmitted, through the differential, from the rear axles to the front axles. Under some operating conditions, the transmission of negative torque as described above would not pose any particular problem. However, whenever the vehicle is in a gradual turn, such as on a ramp leading to or from a freeway, and on a low coefficient-of-friction surface, the occurrence of a negative torque is likely to result in loss of traction at the rear wheels, thus making the vehicle unstable, and the rear wheels likely to spin out.
In order to prevent the transmission of negative torque through the center differential, while the vehicle is moving in a forward direction, it is known to utilize some form of one-way clutch in the driveline. Although the provision of a one-way clutch in the driveline solves the negative torque problem by preventing the transmission of a negative torque from the rear wheels to the front wheels when the vehicle is moving forward, the presence of the one-way clutch prevents the transmission of a positive torque, from the input (front wheels) of the differential to the output (rear wheels), when the vehicle is operating in reverse. In other words, although it would be possible for the vehicle to move in a reverse direction, i.e., to "backup", if the vehicle is of the four wheel drive type, the vehicle would have to rely on the drive torque of the front wheels to do so.
In order to provide drive to the rear wheels, it is known in the vehicle driveline art to provide some means to defeat or bypass the one-way clutch. For example, it is known to engage a jaw clutch whenever the vehicle is shifted into reverse, wherein the jaw clutch is in parallel with the one-way clutch, and effectively bypasses the one-way clutch, such that the driveline transmits positive torque through the jaw clutch when the vehicle is in reverse gear.
In one known system, which is in commercial use, there is provided, between the center differential and the rear axles, an assembly comprising a one-way clutch and a speed sensing, flyweight actuated, sliding clutch, comprising a path in parallel with the one-way clutch. The cited prior art system, being in the driveline downstream of the center differential, must transmit substantially all of the driveline torque, and therefore, must be constructed in a manner that adds substantially to the overall weight and cost of the driveline system.